1. Field of the Invention
This invention relates generally to piezoelectric transducers and more particularly to a wideband ultrasonic transducer employing piezoelectric transducer elements.
2. Description of the Prior Art
Wideband ultrasonic transducers are generally well known in the fields of medical diagnostics, non-destructive materials testing and underwater echo ranging. Many such transducers employ piezoelectric materials that are stimulated with electrical signals to produce ultrasonic vibrations. Some transducers employ a ceramic piezoelectric material such as lead zirconate titanate (PZT). Others employ piezoelectric polymer materials, such as polyvinylidene fluoride (PVDF) or a co-polymer of polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE).
Recently, ultrasonic transducers have found new applications in ultrasonic hearing aids. An ultrasonic hearing aid provides a deaf person with an auditory sense by transmitting ultrasonic waves through a patient's body tissue to the auditory organs. The amplitude of the ultrasonic waves is then modulated by normal sounds in the human auditory range 200 Hz-4 kHz). While deaf persons do not have sensory perception in the normal auditory range of 200 Hz to 4 kHz, it has been found that they often do have perception in the ultrasonic range, and therefore, the modulated ultrasonic waves are perceived by the auditory organs.
Humans, however, are incapable of discerning small frequency variations in the ultrasonic range. Therefore, with ultrasonic hearing aids, the spectrum of audible sounds (200 Hz -4 Khz) must be broadened to cover a broader frequency range prior to modulating those sounds on the ultrasonic waves. Consequently, the ultrasonic transducer supplying the ultrasonic waves must have a correspondingly wide bandwidth. It has been found that a desirable bandwidth for such an ultrasonic transducer is about 20 kHz at a center frequency of about 35 kHz. Unfortunately, piezoelectric transducers typically do not have such wide bandwidths.
However, several techniques are known for broadening the bandwidth of such transducers. For example, one technique for broadening the bandwidth of an ultrasonic transducer is to employ an impedance matching material or layer between the transducer and the radiation medium. As mentioned in U.S. Pat. No. 4,604,542, however, the matching layer must conform to the surface and completely cover the transducer, which makes production more difficult. Also, the thickness of the matching layer has to be a quarter of the wavelength of the material of the matching layer, which restricts the range of operating frequencies in which this technique can be used.
Another technique for obtaining a wide bandwidth device is to employ a plurality of transducer elements, each of which has a different resonant frequency. When operated simultaneously, the individual bandwidths of each transducer element combine to form a wider contiguous frequency band. For example, U.S. Pat. No. 4,916,675 discloses a wideband transducer employing such a technique. The transducer of the '675 patent comprises a plurality of transducer rings positioned side-by-side along a common axis. Each ring consists of a plurality of individual radially directed transducer elements located side-by-side around the circumference of the ring. The individual transducer elements are of the Tonpilz type which comprise a stack of piezoelectric oscillating members positioned between a resonant mass and a counter mass. The resonant frequency of the transducer elements of each transducer ring differs from the resonant frequency of the transducer elements of adjacent rings. The resonant frequencies are spaced such that the bandwidths of each transducer ring combine to cover a wide frequency band.
Similarly, U.S. Pat. No. 4,633,119 discloses a wideband longitudinal transducer comprising a laminar head mass section coupled to electromechanical transducer elements. The head mass section includes a forward head mass, a compliant member abutting the forward head mass and a rear head mass abutting the compliant member and the transducer elements. The compliant member allows the head mass section to mechanically resonate in at least two frequencies thereby expanding the bandwidth of the transducer.
Unfortunately, both the wideband transducer of the '675 patent and the wideband transducer of the '119 patent are complex devices requiring significant manufacturing efforts. Additionally, these transducers were not designed for transmission of ultrasonic waves through human tissue, and their physical geometries preclude such uses. Furthermore, they are not easily adapted to cover different desired frequency bands. There is a need, therefore, for a wideband ultrasonic transducer suitable for sending ultrasonic waves through body tissue with a bandwidth of about 20 kHz. Additionally, there is a need for a wideband transducer having these characteristics that is also easy to manufacture and that is easily adaptable to cover different frequency bands. The present invention satisfies these needs.